1. Field of the Invention
The present invention relates to an image forming apparatus that cools a sheet that has passed through a fixing portion, and specifically relates to an image forming apparatus capable of stably providing an image with a desired gloss level and no gloss unevenness.
2. Description of the Related Art
Conventionally, there are image forming apparatuses such as printers, facsimile machines, copiers and multi-functional machines having functions of these types of machines, and as an example of such image forming apparatuses, there is an image forming apparatus using electrophotography.
In an image forming apparatus using electrophotography, when forming an image on a sheet, an electrostatic latent image is formed on a surface of an image bearing member such as a photosensitive drum by means of a known electrophotographic process, and then, the electrostatic latent image is developed with toner to form a toner image. Next, the toner image is electrostatically transferred onto a sheet such as recording paper directly or via an intermediate transfer member, and the transferred toner image is fixed on the sheet by means of a fixing device. The sheet after passing through the fixing device is nipped and conveyed by, e.g., a conveying roller pair including a driven roller and a conveying roller and ejected to the outside of the apparatus.
In recent years, toner containing, e.g., wax and having an enhanced releasing property from a fixing roller is used. Where toner contains wax, when the toner transferred on a sheet is fused by a fixing device, the wax in the toner is also fused at the same time and thereby interposed between the toner surface and the fixing roller surface, enabling enhancement of the releasing property.
When the toner on the sheet is fused by the fixing device, the wax is precipitated on the surface layer of the toner. Since the wax has a fusing point of around 70° C., which is lower than that of the toner, the wax enters a liquid state on the sheet immediately after passing through the fixing device. Even if the wax enters a liquid state, the wax is subsequently cooled and solidified as a result of, e.g., natural heat dissipation before the sheet is ejected to the outside of the apparatus.
As a method for enhancing the gloss level in an image forming apparatus using wax-contained toner, there is a method in which a sheet is rapidly cooled by means of a cooling device immediately after passing through a fixing device so that wax is solidified before the sheet is nipped by a conveying roller pair coming after the fixing device (see U.S. Pat. No. 7,421,237). As a result of the sheet being rapidly cooled by the rapid cooling device, crystallization of wax is suppressed to enhance the gloss level. In such an image forming apparatus, the conveying roller pair coming after the fixing device is made to be a pair of tubular rollers each having a length larger than the entire width of a sheet to avoid the conveying roller pair from partially nipping the sheet, thereby eliminating gloss unevenness and enhancing the gloss level of the entire surface of the sheet.
Furthermore, as a method for changing the gloss level, there is a method in which a cooling unit for a fixing member is provided in a fixing device to change a controlled temperature of the fixing device, thereby changing the gloss level of the sheet (see Japanese Patent Application Laid-Open No. 2004-085882).
In such a conventional image forming apparatus, while enhancement of the gloss level of the entire surface is often desired in general in the case of, e.g., a photographic image, contrarily an image with a suppressed gloss level is often desired in the case of, e.g., a document image.
In order to provide a low gloss level, it is necessary to gradually cool the sheet to promote crystallization of the wax, thereby suppressing the gloss level. However, such a method requires precise control of the temperature of the sheet, resulting in complexity of the entire apparatus. Furthermore, in the case of the aforementioned method in which a sheet is rapidly cooled to enhance the gloss level of an image, in order to gradually cool the sheet, it is necessary to decrease the speed of conveying the sheet, significantly impairing the productivity of the image forming apparatus.
As stated above, in the case of the method in which the gloss level of a sheet is changed by focusing on the crystallization speed of wax, the gloss level largely changes depending on the surface condition of the sheet when the wax is solidified. In other words, where wax is solidified in a state in which the wax is in contact with, e.g., a roller nip with a flat and smooth surface, the wax surface becomes flat and smooth according to the flat and smooth surface property of the roller, enhancing the gloss level. However, where the wax surface is cooled in an unconstrained state by means of, e.g., air-cooling of the sheet, the wax surface has irregularities subject to the influence of irregularities of the toner and the sheet, resulting in a decrease in the gloss level.
Furthermore, in the case of the method in which a controlled temperature of a fixing device is changed by a cooling unit, the gloss level can be changed by changing the fixing temperature. However, if a low gloss level is provided to a sheet with a large basis weight or an image with a large amount of toner, insufficient fixing of toner may occur because of a decrease in the fixing temperature. Furthermore, toner and wax on a sheet that has just passed through a fixing device are still in a fused state, and thus, neither the influence of the surface property of a conveying roller, nor the influence of a cooled state of the sheet is taken into consideration, reducing the possibility of the occurrence of gloss unevenness. As stated above, the conventional image forming apparatuses have difficulty in stably providing an image with a desired gloss level.